Nonlinear optics open up a wide range of applications for the generation of new frequencies and the control of light by light. The optical power required for nonlinear optical devices can be substantially reduced by using optical waveguides. The applications of guided wave nonlinear optics include second harmonic generation, parametric devices, modulators, and nonlinear switching. The process involves the mixing of one or more optical beams over some interaaction length. The interaction efficiency is governed by the nonlinear optical response of the medium, the intensity of the interacting beams, and the distance over which phase matched mixing occurs. The first factor is a materials issue. The other two factors are optimized by guided wave geometries. Optical waveguides consist of regions of high refractive index bounded by regions of low index, thus providing strong beam confinement over long propagation distances. The size of the confinement regions is the order of the wavelength of light and strong depends on the difference in refractive index between the nonlinear opticl film and the bounding media. The distance over which the mixing occurs is limited by the propagation lobbed, that are closely related to crystal imperfection and interface structures, and by the waveguide nonuniformity.
LiNbO.sub.3 is an attractive material for applications of nonlinear optics because of its large nonlinear susceptibilities, transparency from 350 to 4000 nm, and well developed waveguide technologies. Several methods are known to form a thin film optical waveguide using LiNbO.sub.3. Ti-doped LiNbO.sub.3 films on LiNbO.sub.3 substrates or LiNbO.sub.3 films on Mg-doped LiNbO.sub.3 substrates have been used to form structures with different refractive indices. Although a good epitaxial structure can be achieved in this case due to nearly equal lattice constants of pure and doped-LiNbO.sub.3, the differences in refractive index between the surface layer and underlaying substrate are quite small. For instance, the refractive index of 5-mol% MgO-doped and undoped LiNbO.sub.3 at 630 nm is 2.192 and 2.203, respectively. See "LiNbO.sub.3 thin-film optical waveguide grown by liquid phase epitaxy and its application to second-harmonic generation", by H. Tamada et al., J. Appl. Phys. 70, 2536 (1991). Thus a thick surface layer is required to form a waveguide, and the optical confinement is relatively poor, resulting in low guided-wave intensities and limited efficiencies of nonlinear optical interactions. Recently c-oriented epitaxial films of LiNbO.sub.3 and LiTaO.sub.3 have been grown on sapphire. See "Epitaxial Growth of LiNbO.sub.3 -LiTaO.sub.3 Thin Films on Al.sub.2 O.sub.3 " by T. Kanata et al., J. Appl. Phys. 62, 2989 (1987). In such cases a significant improvement in optical confinement can be achieved, because the Al.sub.2 O.sub.3 refractive index 1.76 is substantially lower than those of LiNbO.sub.3 or LiTaO.sub.3. However, the large lattice mismatch of 7.6% for LiNbO.sub.3 or LiTaO.sub.3 grown on Al.sub.2 O.sub.3 results in relatively poor crystallines and surface morphologies, as compared to the first approach. U.S. Pat. No. 5,158,823 issued Oct. 27, 1992 to Enomoto et al achieved a second harmonic wave generating device using sequential deposition of LiTaO.sub.3 and LiNbO.sub.3 on LiNbO.sub.3 single crystals to form a structure of LiNbO.sub.3 /LiTaO.sub.3 /LiNbO.sub.3. Lattice matching is achievable because of the similar crystal structure of LiNbO.sub.3 and LiTaO.sub.3, and the difference in refractive index is approximately 0.1, substantially greater than that obtained by the first approach. However, the difference in refractive index is not sufficiently high to achieve the very tight optical confinement. Moreover, the concept is not applicable to the use of LiTaO.sub.3 for nonlinear optical devices, because the reversal of the deposition sequence with LiTaO.sub.3 on LiNbO.sub.3 forms a non-waveguide structure.